1. Field of the Invention
This invention relates to a method of producing a Raney type thin layer catalyst metallurgically combined with the surface of a structure.
2. Description of the Prior Art
Raney catalysts find extensive applications as catalyst for hydration reactions of ethylenic linkage and methanation reaction. The Raney catalyst is manufactured by forming an alloy of a catalytic metal such as nickel, cobalt, chromium and iron and an active metal such as aluminum, magnesium and silicon and then dissolving the active metal by means of an acid or alkali solution. The most common Raney catalyst is manufactured by pulverizing nickel-aluminum alloy with a nickel content of 30 to 50 weight percent into powder of a grain size of 150 to 200 mesh and immersing the powder in a caustic soda solution of 3 normal concentration at 50.degree. C for developing aluminum.
This catalyst is in the form of porous particles having dimensions on the order of atomic size. In use, it is charged in a reactor and provides catalytic activity for the reaction gas passing through the charged layer.
The most typical methanation reaction is one represented by CO + 3H.sub.2 .fwdarw. CH.sub.4 + H.sub.2 O - .DELTA.H = 49 kcal/mol. Since this reaction is an exothermic reaction, the heat of reaction has to be removed to control the temperature. Also, since the volume of the product system is one half that of the reaction material system, the pressure has to be increased to increase the reaction speed. Accordingly, the ordinary methanation reaction is carried out under high temperature high pressure conditions of 300.degree. to 600.degree. C and 20 to 70 atm.
Where the afore-mentioned powder layer is used as the catalyst, the removal of heat is very difficult. Accordingly, a dilution method is adopted as a means for repressing the rise of temperature due to reaction heat. The dilution method is one in which part of the product gas is fed back to the material gas to thereby dilute the material gas. Increasing the partial pressure of methane in the gaseous phase reaction system a simultaneous leads to increase in the heat capacity of the gaseous phase and to a slippage in the balance of reaction, thus restricting the production of methane and therefore preventing a sudden temperature rise due to the reaction heat. However, in order to restrict the reaction by this method about 90 percent of the product gas has to be recirculated, thus leading to a reduced yield and also to an extreme increase in the size of the reactor system.
It has been proposed to carry out the methanation reaction by using a heat exchanger type reactor having the same construction as a heat exchange with a view to maintaining a low reaction temperature for increasing the rate of conversion into methane by directly removing the heat of reaction from the reaction system. For example, this method resorts to a catalyst which is constituted by the reactor tube wall.
Heretofore, the tubular catalyst consisting of Raney nickel has been formed by coating powdery nickel-aluminum alloy by sprayed metal coating over the outer surface of a tube of a different material and subsequently developing aluminum through treatment with alkali.
Since the methanation reaction is a high temperature and high pressure reaction, however, a methanation reactor using as tubular catalyst a reactor tube provided with a catalyst layer formed on the outer surface of the tube is disadvantageous in that the whole apparatus accommodating the reactor tube, the outer surface of which is exposed to high temperature and high pressure, must have a temperature-resistant pressure-bearing structure so that the cost of the apparatus is high. In addition, since the methanation reaction is an exothermic reaction producing a great deal of heat, the reaction heat has to be removed as efficiently as possible. However, in the prior art construction where the reactor tube has a catalyst layer on its outer surface and is cooled on its inner, the reaction area is greater than the cooling area and sufficient heat exchange therefore cannot be obtained.
Further, in the sprayed metal coating of a high aluminum alloy such as Raney nickel, preferential oxidation aluminum having a greater capability of oxidation takes place to result in the formation of aluminum oxide at the alloy surface. Thus, the peel-off resistance of the sprayed metal coating layer is extremely low, that is, the catalyst layer formed on the outer surface of the tube by the sprayed metal coating method is very likely to peel off.
If the catalyst layer is formed on the inner surface of the tube, the cooling efficiency and peel-off resistance can be increased compared to the case of an outer surface catalyst layer. However, it is very difficult to form a sprayed metal coating layer on the inner surface of a tube of a diameter less than 50 millimeters with a sprayed metal coating torch because of the shape thereof. In cases where it is possible, manufacturing costs are high.
In the meantime, it has been proposed to form a catalyst layer on the inner surface of a tube by coating the inner surface with a developable metal by means of electroplating, vapor depositions or immersion in the molten metal, subsequently diffusing the developable metal into the catalytic metal through heat treatment and then developing the catalyst layer. By this method, however, a uniform catalyst layer cannot be obtained unless a residual layer of non-diffused developable metal is removed prior to the development, and the removing operation is very troublesome. Further, failure of diffusion is likely to result depending upon the state of boundary between the catalytic metal layer and developable metal layer. Thus, it is very difficult to form a uniform catalyst layer on the inner surface of a tube of long length.